RT8205M datasheet(19/26 Pages) RICHTEK

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RT8205L/M

DS8205L/M-05 June 2011

www.richtek.com

ILIMx

DS(ON)

V(R

10 A)/10 I

=×

ILIMx

DS(ON)

R(I

) 10 10 A

=×

Carefully observe the PC board layout guidelines to

ensures that noise and DC errors do not corrupt the current

sense signal at PHASEx and GND. Mount or place the IC

close to the low side MOSFET.

Charge Pump (SECFB)

The external 14V charge pump is driven by LGATEx (Figure

3). When LGATEx is low, C1 will be charged by D1 from

VOUT1. C1 voltage is equal to VOUT1 minus a diode drop.

When LGATEx transitions to high, the charges from C1

will transfer to C2 through D2 and charge it to VLGATEX plus

VC1. As LGATEx transitions low on the next cycle, C2

will charge C3 to its voltage minus a diode drop through

Figure 2. “ Valley” Current Limit

The RT8205L/M uses the on resistance of the synchronous

rectifier as the current sense element and supports

temperature compensated MOSFET RDS(ON) sensing. The

RILIMx resistor between the ENTRIPx pin and GND sets

the current limit threshold. The resistor RILIMx is connected

to a current source from ENTRIPx, which is typically10

μA

at room temperature. The current source has a 4700ppm/

°C temperature slope to compensate the temperature

dependency of the RDS(ON). When the voltage drop across

the sense resistor or low side MOSFET equals 1/10 the

voltage across the RILIMx resistor, positive current limit

will be activated. The high side MOSFET will not be turned

on until the voltage drop across the MOSFET falls below

1/10 the voltage across the RILIMx resistor.

Choose a current limit resistor by following equation :

gate driver waveform to become the complement of the

high side gate driver waveform. This in turn causes the

inductor current to reverse at light loads as the PWM loop

to maintain a duty ratio of VOUT/VIN. The benefit of forced

CCM mode is to keep the switching frequency fairly

constant, but it comes at a cost. The no-load battery

current can be from 10mA to 40mA, depending on the

external MOSFETs.

Reference and Linear Regulators (REF, VREGx)

The 2V reference (REF) is accurate within

±1% over the

entire operating temperature range, making REF useful

as a precision system reference. Bypass REF to GND

with a minimum 0.22

μF ceramic capacitor. REF can supply

up to 100

μA for external loads. Loading REF reduces the

VOUTx output voltage slightly because of the reference

load regulation error.

The RT8205L/M includes 5V (VREG5) and 3.3V (VREG3)

linear regulators. The VREG5 regulator supplies a total of

100mA for internal and external loads, including the

MOSFET gate driver and PWM controller. The VREG3

regulator supplies up to 100mA for external loads. Bypass

VREG5 and VREG3 with a minimum 4.7

μF ceramic

capacitor.

When the 5V main output voltage is above the VREG5

switchover threshold (4.75V), an internal 1.5

Ω P- MOSFET

switch connects VOUT1 to VREG5, while simultaneously

shutting down the VREG5 linear regulator. Similarly, when

the 3.3V main output voltage is above the VREG3

switchover threshold (3.125V), an internal 1.5

P-MOSFET switch connects VOUT2 to VREG3, while

simultaneously shutting down the VREG3 linear regulator.

It can decrease the power dissipation from the same

battery, because the converted efficiency of SMPS is

better than the converted efficiency of the linear regulator.

Current Limit Setting (ENTRIPx)

The RT8205L/M has a cycle-by-cycle current limit control.

The current limit circuit employs an unique “Valley” current

sensing algorithm. If the magnitude of the current sense

signal at PHASEx is above the current limit threshold,

the PWM is not allowed to initiate a new cycle (Figure 2).

The actual peak current is greater than the current limit

threshold by an amount equal to the inductor ripple current.

Therefore, the exact current limit characteristic and

maximum load capability are functions of the sense

resistance, inductor value, and battery and output voltage.

IL, peak

ILIM

ILoad

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